Tape cartridges in use today incorporate a magnetic tape having data tracks for recording a succession of units of information known as data sets. Each data set consists of a data region within which data is recorded and a data set information table that describes the content of the data region. Data to be recorded is sent to the cartridge by a tape drive and is recorded within the data regions. The data set information table for each data set has a multiplicity of fields each of which has a pre-allocated number of bytes of storage. The fields store records of information such as the data set number, the valid data length, the data set type, and the drive manufacturer identity. The data sets are also encoded with a tape write pass value. The write pass value is set to 1 when the tape is first written to. The write pass value is incremented at certain logical points during the writing of data sets along the tape including a point where a change is made in the physical forward or reverse direction of movement of the tape or at a point where an append operation begins and causes an existing data set to be overwritten.
Data written to the tape may be protected from being overwritten. Such data is protected by drive level processing that renders the tape as write-once-read many (WORM) protected. When trying to protect data written to tape in a WORM environment, safeguards are implemented to prevent the data from being overwritten, accidentally or intentionally. One of the main considerations with WORM protected data is to be able to detect whether the data has been tampered with by overriding the safeguards designed to protect the data.
One such safeguard to prevent overwriting of WORM protected data includes providing information in a cartridge memory to inform the drive that the cartridge is WORM protected. It may, however, be possible to tamper with the cartridge so that either information in the cartridge memory is altered or the cartridge memory is swapped for another such memory, thereby allowing the unauthorised overwriting of data that should be WORM protected.
EP 0 936 618, incorporated herein by reference, describes a method of encoding data on a data storage tape in a manner to distinguish between new data and incompletely erased old data. The data is arranged in track blocks each having a track block header including a write pass identifier field initially set to 1 on a first occasion that data is written. On each subsequent occasion that data is written to cause existing data to be overwritten, the write pass identifier field is incremented by 1. If the write pass identifier field of any particular block is found to be at a lower value than that of the preceding blocks, then that block is determined to be from old, incompletely overwritten data.